Stabilizing laminate inserts for resin transfer molding

ABSTRACT

A preform assembly for use with a resin transfer molding apparatus. The preform assembly includes a dry fabric preform and a thin precured fiber-reinforced resin impregnated laminate, the preform being folded about and firmly fastened to said laminate to form a preform assembly having a stiffened and stabilized preform edge, wherein the edge aids in the insertion of the preform assembly into a mold cavity or channel of a resin transfer mold apparatus. Methods for making such preform assembly and composite article are also disclosed.

This invention was made with Government support. The Government hascertain rights in this invention.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to commonly assigned U.S. application Ser.No. 07/399,678 filed on even date herewith entitled "Stabilizing FoamInserts for Resin Transfer Molding".

TECHNICAL FIELD

The present invention relates to resin transfer molding and, morespecifically, to a means of loading preform assemblies into a resintransfer mold apparatus.

BACKGROUND ART

Resin transfer molding is a closed mold, low pressure process applicableto the fabrication of complex, high performance composite articles ofboth large and small size. Several different resin transfer moldingprocesses are well known to those skilled in the art. The processdiffers from various other molding processes in that a reinforcingmaterial or preform such as glass fibers or other fiber reinforcement,is placed separately into a mold tool cavity. Resin is then injectedunder pressure into the mold cavity to combine with the preform to forma fiber reinforced plastic composite product.

Typically, a pre-shaped fiber reinforced preform is positioned within amolding tool cavity and the molding tool is closed. The mold issubsequently evacuated by pulling a vacuum through the mold. A feed lineconnects the closed molding tool cavity with a supply of liquid resinand the resin is pumped or "transferred" into the cavity where itimpregnates and envelopes the fiber reinforced preform and subsequentlycures. The cured or semi-cured product is then removed from the moldingtool cavity.

The primary advantage of resin transfer molding resides in its capacityfor high rate production. Although this process is widely known, the useof this molding process has not become widespread because of problemsassociated with the process. For example, use of the process has beenhampered by the difficulties associated with stabilizing and de-bulkingthe dry composite preform and loading the same into the mold cavity.Maintaining adequate tolerances of the components and sealing the moldapparatus is also problematic.

Specifically with regard to molding aerospace structures, thesedifficulties often yield disoriented fibers, areas which are resin richor lean depending upon bulk variations of a preform assembly, orporosity (i.e. trapped air due to an improper vacuum) within thecomposite which greatly diminishes the specific strength of thecomponent. Due to the high costs of both fabrication and material,rejection of a single part out of a batch can negate the savingsinitially. As a result, use of the process has typically been limited torather simple, low-strength components as compared to high strengthaerospace components.

Thus, there is a continuing need in this field of art for means toobviate the above problems.

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to provide astabilization technique which facilitates the handling and loading ofpreform assemblies into a resin transfer mold apparatus.

Another objective is to produce an improved component of increasedstrength and allow for net molding of same.

The present invention utilizes a preform assembly for use in a resintransfer mold apparatus. More specifically, the present inventioncomprises a dry fabric preform and a thin precured fiber-reinforcedresin impregnated laminate. The preform is folded about the laminate toform a preform assembly having a stiffened and stabilized preform edge.The edge aids in the insertion of the preform assembly into a moldcavity or channel of the resin transfer mold apparatus.

A further aspect of the disclosure includes a method for preparing apreform assembly which includes cutting a dry fabric preform to thedesired shape and thickness, forming a thin precured fiber-reinforcedresin impregnated laminate to typically conform to the desired preformshape, placing the laminate over the preform, folding the preform abouta fold line so that the laminate is surrounded by the preform, andfirmly fastening the preform to the laminate to form a preform assemblyhaving a stiffened and stabilized preform edge section.

A still further aspect of the disclosure includes a method for making acomposite article which includes forming a preform assembly, asdisclosed by the present invention, inserting an edge of the preformassembly into a mold cavity of a resin transfer molding apparatus,tapping the preform assembly to fully occupy the mold cavity, closingthe molding apparatus, introducing resin into the cavity to form acomposite article, and removing the article from the molding apparatussubsequent to curing and cooling the article.

The foregoing and other features and advantages of the present inventionwill become more apparent from the following description andaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts an exploded perspective view of a prior art mold body anda prior art preform.

FIG. 2 illustrates a perspective view of the dislodged and disorientedfibers, as well as areas of unreinforced resin, associated withconventional resin transfer mold processes.

FIG. 3 illustrates a perspective view of the laminate used in thepresent invention.

FIG. 4 illustrates a perspective view of a partly assembled laminate anddry fabric preform, and their relationship thereto, of the presentinvention.

FIG. 5 illustrates a perspective view of an assembled laminate and dryfabric preform to yield the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A prior art resin transfer mold and preform are depicted in FIGS. 1 and2. According to FIG. 1, a dry fabric preform 10, typically referred toas a stringer, is loaded into a channel or mold cavity 12 of a mold body8. Due to the need to maintain high fiber volume, the mold cavity 12 isgenerally smaller in volume than the bulky stringer 10, despite thestitching 11 which provides some debulking and stabilization to thestringer 10. Hence, it is difficult to ensure that the entire shear web14 fully occupies the mold cavity 12.

Problems with the prior art are more clearly visible in FIG. 2. In FIG.2, areas or pockets of unreinforced resin 16 are formed as a result ofthe unfilled sections of the mold cavity 12 with the shear web 14.Additionally, dislodged and disoriented fibers 18 are often a result ofresin injection pressures and resin flow. These fibers 18 severelydegrade the strength of the composite component.

A clearer understanding of the present invention may be made byreference to FIGS. 3, 4 and 5. In FIGS. 3 and 4, a fiber reinforcedresin impregnated laminate 22 and 22' is depicted. As illustrated, thelaminate may be homogeneous 22 or perforated 22' (to aid in thecapillary or wicking action of the resin).

A dry fabric preform 20 is illustrated in FIG. 4. The preform 20comprises a plurality of fabric patterns cut to the desired twodimensional shape and stacked to yield the desired preform thickness.

The laminate 22, 22' is placed over the preform 20 adjacent to a foldline 24. The preform 20 is then folded about the fold line 24 so thatthe laminate 22, 22' is surrounded by the preform 20. The laminate 22,22' is firmly fastened to the preform 20, by a means such as stitching11, to form a preform assembly having a stiffened and stabilized preformedge section 26.

The edge section 26 is inserted into the mold channel or cavity 12. Thesection occupies the entire depth of the cavity and may be hand ormallet tapped into place.

More specifically, the laminate is a thin fiber-reinforced resinimpregnated composite. It is precured to impart rigidity to it, as wellas serving as a stabilizer to the preform. This rigidity alsofacilitates the shaping of the laminate to typically conform to theshape of the preform and ultimately, the composite article. The fibersand resins used in the laminate are those conventional to the art suchas glass, polyaramid, graphite, epoxies, polyester, vinyl esters,phenolics and urethanes.

Preferably, the laminate is compatible to the resin to be injected intothe mold. By compatible is meant that the insert maintains itsmechanical and thermal properties and is able to withstand the pressuresand temperatures exerted during the molding process. Preferably, thelaminate has a thickness that will provide suitable rigidity tostabilize the preform and ensure an improvement in mechanicalproperties, but is still flexible enough to maintain any contours in thelaminate. Depending on the thickness of the preform and the width of themold cavity, the laminate will typically have a thickness between about0.005 inches and about 0.01 inches.

In addition, the laminate preferably has a height dimension such that itwill be accessible from the top of the mold cavity when placed in themold cavity. This allows the laminate to be firmly placed within thecavity as further described herein.

The dry fabric preform comprises a plurality of fabric patterns cut tothe desired two-dimensional shape. Preferably, the preform is untreatedto ensure optimal strength. These patterns are then stacked upon eachother to yield a desired preform thickness. The fabric preform used inthe present invention are also made from materials conventional to thosein the art, such as E glass fibers (for low modulus applications), Sglass fibers (for high modulus applications), graphite fibers, aramidfibers including KEVLAR (™) polyamide fiber (E.I. Dupont de Nemours,Wilmington, Del.), and the like. For use in the preforms, such fibrousreinforced material can be used in any of various configurationsincluding, for example, random chopped fiber, continuous random fiber,and oriented continuous fiber, for example, nonwoven, woven, knitted,braided, etc.

The laminate is placed over the preform adjacent to a fold line, suchfold line being the symmetrical center of the preform. The preform isfolded about and surrounds the laminate. The preform is firmly fastenedto the laminate to form a preform assembly having a stiffened andstabilized preform edge section. The preform assembly has a combinedthickness (laminate and preform itself) or cross sectional area that isslightly larger than the channel or cavity width of the particular moldapparatus. By this is meant that the preform assembly will fit snuglyinto the channel or cavity but not so tight that the dry fabric preformwill bunch or gather.

The preform assembly for use in a resin transfer mold apparatus isprepared by cutting a dry fabric preform to the desired shape, thepreform comprising a plurality of two-dimensional fabric patternsstacked to the desired thickness. A thin precured fiber-reinforced resinimpregnated laminate is formed, by methods known to one skilled in theart, to typically conform to the desired preform shape. The laminate isthen placed over the preform adjacent to a fold line, the fold linebeing the symmetrical centerline of the preform. The preform is foldedabout the fold line, surrounding the laminate. Subsequently, the preformis firmly fastened to the laminate to form a preform assembly having astiffened and stabilized preform edge section. By firmly fastened ismeant that the laminate is held in a fixed relation to the preform. Thisfastening is typically achieved by methods known to one skilled in theart. For example, the preform assembly may be stitched together with athread of polyester, KEVLAR (™), etc., or may be adhesively attached byusing a hot iron.

The stabilized edge section is inserted into the mold channel or cavityby hand. The top of the preform assembly is hand or lightly mallettapped into place to ensure that the cavity or channel will be fullyoccupied by the preform assembly. This may also be accomplished by usingthe cover of the resin transfer mold or any other suitable means. Byfully occupied is meant that the stabilized section edge occupies theentire depth of the mold cavity and is flush against each side of thecavity, i.e. the sides and bottom. Suitable precautions are preferablytaken to avoid any possible shearing to the fabric.

The dry fabric preform may also be open-faced at its fold line. Thisembodiment is foreseeable/desirable when the closed wrapping of thepreform is not needed for extra structural performance. Necessary stepsshould be taken to ensure that the now opened laminate is locked, i.e,firmly held in place, with the preform at the fold line. A lock stitchor the like is suitable.

While a T-shaped preform assembly is depicted in FIG. 5, it will beapparent that the teaching of the present invention is applicable to avariety of preform shapes, such as flat surfaces, circular surfaces,etc.

When the mold is fully assembled, the resin is transferred underpressure into the evacuated article chamber impregnating the dry fabricpreform and laminate assembly. The resin is forced into the mold underconventional pressures. These typically range from about 85 psi to about300 psi. Once the mold is full, the resin impregnated preform assemblyis exposed to conventional pressures and temperatures appropriate forthe particular resin and fiber used. Typically, these pressures andtemperatures are about 5 psi to about 300 psi and about ambient to about500 F. Once fully cured, the mold can be disassembled and the fullycured article may be removed.

The use of the preform assembly not only provides a stabilizing means tothe component article, but also aids in the insertion of such assemblyinto a mold cavity. This preform assembly provides a substantialimprovement to other preforms available in the art because only aminimal, if not negligible, amount of dislocation and distortion of thepreform fibers associated with the resin injection process occurs. As aresult, net molding of composite articles becomes more feasible as thetrimming process is substantially eliminated. Overall, a more efficientand desirable process for producing resin transfer mold high strengthcomponents is created.

Although the invention has been shown and described with respect to apreferred embodiment, it will be understood by those skilled in the artthat various changes in form and detail thereof may be made withoutdeparting from the spirit and scope of the claimed invention.

What is claimed is:
 1. A preform assembly for use with a resin transfermolding apparatus which comprises:(a) a dry fabric preform; and (b) athin fiber-reinforced resin impregnated laminate, wherein said laminateis precured for imparting rigidity to said laminate; (c) said preformfolded about said laminate and said preform firmly fastened to saidlaminate to form a preform assembly having a stiffened and stabilizedpreform edge;wherein said edge aids in the insertion of said preformassembly into a cavity of a resin transfer mold apparatus.
 2. Thepreform assembly according to claim 1 wherein said dry fabric preformcomprises a plurality of fabric patterns having a desiredtwo-dimensional shape and thickness.
 3. The preform assembly accordingto claim 1 wherein said preform comprises material selected from thegroup consisting of E glass fibers, S glass fibers, graphite fibers andaramid fibers.
 4. The preform assembly according to claim 1 wherein saidlaminate is perforated.
 5. The preform assembly according to claim 1wherein said laminate is homogeneous.
 6. The preform assembly accordingto claim 1 wherein the combined thickness of said preform and saidlaminate to form said assembly is such that said assembly fits snuglyinto said mold cavity.
 7. A method for preparing a preform assembly foruse in a resin transfer mold apparatus which comprises the steps of:(a)cutting a dry fabric preform to a desired shape, said preform comprisinga plurality of fabric patterns stacked to the desired preform thickness;(b) forming a thin fiber-reinforced resin impregnated laminate to matchsaid desired preform shape, wherein said laminate is precured forimparting rigidity to said laminate; (c) placing said laminate over saidpreform and adjacent to a fold line, said fold line being thesymmetrical centerline of said preform; (d) folding said preform aboutsaid fold line so that said laminate is surrounded by said preform; and(e) firmly fastening said preform to said laminate to form a preformassembly having a stiffened and stabilized preform edge section,whereinsaid edge section aids in the insertion of said preform assembly intosaid resin mold transfer apparatus.
 8. The method according to claim 7wherein said fastening is stitching.
 9. The method according to claim 7Wherein said fastening is an adhesive.
 10. A method for making acomposite article which comprises the steps of:(a) forming a preformassembly, said assembly comprising a dry fabric preform and a thinfiber-reinforced resin impregnated laminate, wherein said laminate isprecured for imparting rigidity to said laminate, said preform foldedabout said laminate and said preform firmly fastened to said laminate toform a preform assembly having a stiffened and stabilized preform edge;(b) inserting said preform edge of said preform assembly into a moldcavity of a resin transfer molding apparatus; (c) tapping said preformassembly to fully occupy said mold cavity; (d) closing said moldingapparatus on said preform assembly; (e) introducing resin into saidcavity of said molding apparatus to form a composite article; and (f)removing said article from said molding apparatus subsequent to curingand cooling said article,wherein said article is a net molded parthaving a minimal amount of dislocation and distortion of said preformfibers.
 11. The method according to claim 10 wherein said insertion isby hand.